Characteristics of proton beams after field shaping at PMRC.

The proton irradiation control system was developed for cancer radiotherapy at the Proton Medical Research Center, with the extension of a beam line connected to a synchrotron at the High Energy Physics Laboratory. The initial energy of the 500 MeV proton beam supplied by the accelerator is degraded down to 243 MeV after passing through a graphite rod. In the control system, a proton beam is scattered to form a large field, its Bragg peak width is spread out, and its energy is degraded to the optimum value with a range covering tumour depth. The characteristics of the devices required for these procedures have been investigated from the viewpoint of the relationship between dose rate and field flatness, taking the setting-up geometry of these devices into consideration.